1. Field of the Invention
The present invention relates to golf ball materials which have a good heat stability, flow and processability and from which there can be obtained high-performance golf balls endowed with excellent properties such as rebound and durability. The invention also relates to golf balls which include as an essential component therein a molding made from such a golf ball material, and to methods for preparing such golf ball materials.
2. Prior Art
In recent years, ionomer resins have been widely used in cover materials for golf balls. Ionomer resins are ionic copolymers of an olefin such as ethylene with an unsaturated carboxylic acid such as acrylic acid, methacrylic or maleic acid, in which some of the acid groups are neutralized with metal ions such as sodium, lithium, zinc or magnesium. These resins provide excellent characteristics in terms of durability, rebound and scuff resistance of the ball.
At present, the base resins used in cover materials for golf balls are generally ionomer resins, but a variety of modifications have been made to address the incessant desire by golfers for golf balls having a high rebound and an excellent flight performance.
For example, to improve the rebound and cost characteristics of ionomer cover materials, U.S. Pat. No. 5,312,857, U.S. Pat. No. 5,306,760 and International Application WO 98/46671 describe cover materials composed of an ionomer resin to which a large amount of a metallic soap has been added.
However, the metallic soap in these cover material undergoes decomposition and vaporization during injection molding, generating a large amount of fatty acid gas. As a result, molding defects tend to arise. Moreover, the gas that forms deposits on the surface of the molding, greatly lowering its paintability. The rebound characteristics obtained with these cover materials do not differ much from those provided by ionomer covers of the same hardness which contain no metallic soap; either the rebound in both cases is about the same or, at most, only a small positive effect is observable from the inclusion of a metallic soap. Hence, such inclusion does not markedly increase rebound. Moreover, depending on the type of metallic soap used, the processability and the rebound characteristics are sometimes greatly compromised, making the cover material entirely unfit for practical use.
An ionomer for use as a golf ball material has recently been developed in the form of a high rebound resilience material which is in a homogeneous phase and has an interpenetrating network (IPN) structure. The ionomer is obtained by mixing a first component such as an ethylene-(meth)acrylic acid copolymer with a different type of thermoplastic resin as a second component to form a resin composition, then adding a metal ionic species as a third component to neutralize the acid on the first component dispersed in the resin composition (U.S. Patent Application No. 2004/0044136). However, in this prior-art production method, because direct use is made of a solid (powder or granular material) such as a metal oxide, metal hydroxide or metal carbonate as the source of the metal ions, and also because a high acid content in the first component necessitates the addition of a large amount of the metal ions required for neutralizing the acid, during admixture of the metal ions, poor dispersion sometimes arises, leaving some of the metal ions unreacted. Moreover, a partial neutralization reaction (incomplete degree of neutralization) occurs, making it impossible to achieve the target degree of neutralization in a one-step reaction within an extruder. Hence, more than one extruder pass is made, which it is feared may lower the physical properties of the resulting ionomer material.